Additives to reduce curl in paper



United States Patent Ofifice Patented Jan. 4, 1966 3,227,578 ADDITIVES TO REDUCE CURL IN PAPER Ernest Malcolm London Parkin, Wealdstone, Harrow,

England, assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Oct. 12, 1961, Ser. No. 144,531 6 Claims. (Cl. 117154) This invention concerns methods for treating paper to reduce or eliminate curl, particularly for treating photographic paper to stabilize the paper against eventual change caused by change in the humidity and the like.

Photographic papers are often coated with gelatin or vinyl compositions which have a diflierent co-efiicient of expansion and contraction from that of the cellulosic materials of which papers are made. Consequently, upon drying or with changes in relative humidity, the papers tend to curl or to change dimensionally. Various means have been suggested for eliminating curl in photographic paper, particularly those directed to treating the gelatin layer or the photographic emulsion, to eliminate the effects caused by contraction or expansion of the surface layer.

Mechanical rollers or flexers have been used to decrease curl, but these sometimes result in myriad small cracks in the coated surface. Steaming the support side of the print and flexing the print while in a steamed condition to reduce the effect of the mechanical treatment is disclosed in US. Patent 1,880,451. Various print flattening solutions have been used which contain chemical compositions such as ethylene glycol, glycerol, diethylene glycol, and the like. These compounds depend upon their hydroscopic nature to accomplish whatever benefit is obtained by their use. Furthermore, they must have an atmosphere of relatively high humidity to be effective.

Most of the methods which have been used for postprocessing curl control have involved treatment of the paper prints immediately following the Washing step in photographic processing. This is often due to the desire to avoid treatment of the paper with reagents which would be removed in the processing solutions or chemicals which might affect the photographic emulsions.

Treatment of the paper base prior to coating with a photographic emulsion to reduce curl has also been suggested. Such methods as applying a gelatin coating to the reverse side of the paper substrate in order to counteract the elfect of the photographic emulsion have been used with some success. Other compensating methods have been tried, such as the coating of other materials on the reverse side, the coating of a discontinuous polymeric coating on the paper before applying the baryta coating with a waterproofing or water resistant layer on both sides to avoid absorption of water, and the like. These treatments have all been relatively expensive and a method has been desired which would be easy to apply, which would be longlasting, effective in eliminating or reducing curl, and which would be photographically inert with respect to photographic emulsion.

I have made the discovery that the curl of paper containing a gelatinous coating can be reduced or eliminated by simply immersing the paper base before coating in a bath containing a chemical agent such as sodium formaldehyde bisulfite, chloral hydrate, or urea.

One object of this invention is to provide a method for eliminating or reducing curl in photographic paper. Another object is to provide a method of treating paper base with a chemical agent, prior to coating with a photographic emulsion or with a baryta layer and a photographic emulsion, to reduce or eliminate curl. An additional object is to reduce the dimensional changes in paper.

The above objects are obtained by immersing the photographic paper base in a 10-15 by weight solution of a chemical agent so that penetration into the base takes place. The quantity of agent picked up by the base should be within the range of 510 grams per square meter. In order to increase or facilitate the absorption into the paper base, methyl or ethyl alcohol may be added to the solution.

After drying, the impregnated base may be coated with baryta or the like. Paper intended for use in the chemical transfer positive process or silver salt diffusion process may also be treated prior to coating the paper with the gelatin layer.

The following examples are intended to illustrate my invention but not to limit it in any way:

EXAMPLE 1 Table I Curl of 10 cms. Circular Disk in ems. Percentage of Chemical in Solution Used For Impregnation After 10 see. Before mono-bath Processing Processing EXAMPLE 2 Paper base as described in Example 1 was also treated with chloral hydrate with the results shown in Table II.

EXAMPLE 3 Paper base as described in Example 1 was also treated with urea with the results shown in Table H.

Table I1 Curl after Curl after Coating Solution used for Impregnating Base Coating (in crns.)

(in ems.) Post-Process Pres-process 10 sec.

mono-bath Distilled Water 5. 6 Not measured 10% Chloral Hydrate in distilled water 2.6 1. 8 10% Urea in distilled water 0.2 2.1

Raw base was soaked in the appropriate solution for 15 minutes, dried under room conditions and subsequently coated with a nucleated gelatin layer of composition given above. The paper was then dried using hot air (in a fiat bed drier) and the curl test was applied.

Formula for coating on to treated raw base:

Samples are conditioned overnight at F. at a relative humidity of 22%. Disks (10 cms. in diameter) are cut from each sample, and holes punched in the center of each disk. The disks are placed on rods inside a dry cabinet which has been maintained at a relative humidity of 20-24% at a temperature of 98-102 F. After conditioning for sixteen hours, a disk to be measured is removed and the distance in cms. measured between the two curled edges. To obtain the values used for comparison purposes the measurement is subtracted from 10 and the averages of four samples are used as the value of a given sample. A fiat sample would have a value of 0 expressed in cms., and a completely cylindrical disk would have a value of 10 cms.

The processing time is desirably short in order to avoid washing out substantial amounts of the anti-curl substance.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.

I claim:

1. A process of treating photographic paper base to reduce curl, comprising impregnating the paper with sodium formaldehyde bisulphite.

2. A process of treating photographic paper to reduce curl, comprising impregnating the paper base with chloral hydrate.

3. The process of treating photographic paper to reduce curl comprising immersing the paper base in a 10-15% solution of sodium formaldehyde bisulphite so that the paper base will contain 5-10 grams per square meter dry weight.

4. The process of treating photographic paper to reduce curl comprising immersing the paper base in a 10-15% solution of chloral hydrate so that the paper base will contain 5-10 grams per square meter dry weight.

5. The process of treating photographic paper coated with a gelatinous layer to reduce curl comprising immersing the paper base prior to coating in a 10-15 solution of sodium formaldehyde bisulphite so that the paper base will contain 5-10 grams pe-r square meter dry weight.

6. The process of treating photographic paper coated with a gelatinous layer to reduce curl comprising immersing the paper base prior to coating in a 10-15 solution of chloral hydrate so that the paper base will contain 5-10 grams per square meter dry weight.

References Cited by the Examiner UNITED STATES PATENTS 2,097,509 11/1937 Boller 117154 X 2,700,621 l/1955 Schwalbe et a1. 117155 X 2,865,745 12/1958 Chan et al. 9685 X 2,965,792 12/1958 Gray 117-60 X OTHER REFERENCES Gregory, T. C.: Uses and Applications of Chemicals, N.Y., Reinhold, 1939, page 629.

Fisher, H. D.; Effect of Urea and Related Compounds on Mechanical Properties of Paper, TAPPI, vol. 34, No. 6, June 1951, pp. 276-288.

WILLIAM D. MARTIN, Primary Examiner.

RICHARD D. NEVIUS, Examiner. 

1. A PROCESS OF TREATING PHOTOGRAPHIC PAPER BASE TO REDUCE CURL, COMPRISING IMPREGNATING THE PAPER WITH SODIUM FORMALDEHYDE BISULPHITE. 